Physiological and clinical studies have demonstrated that the measurement of aortic blood flow can be of great value in characterizing the behavior and performance of the heart and cardiovascular system. A non-invasive, ultrasonic Doppler technique for transcutaneous measurement of blood flow velocities in the aorta has been developed. Important potential applications exist for this technique in continuous monitoring of post operative or other high risk patients; screening of large numbers of people; and measurements in the presence of cardiovascular perturbations such as exercise. It is proposed that this technique be evaluated through a systematic set of clinical trials and that the instrumentation be improved by continued electronic development. Blood flow velocity in both the ascending aorta and the arch can be assessed from the supra-sternal notch by continuous wave ultrasonic Doppler techniques. This information is reliably quantitative, only for the arch. Furthermore, the information is in the form of a complex audio signal spectrum. The development of new signal processing techniques has made possible a simple and inexpensive instrument for obtaining such velocity information directly and in real time. Evaluation of the technique is planned in order to determine the extent to which it is applicable to all patients, its accuracy, and the clinical usefulness of the data which it produces. Trials will be conducted on normal subjects, patients at rest, patients recovering from surgery or myocardial infarction, patients undergoing simultaneous angiography, and patients and normal subjects during cardiovascular stress, such as exercise. Refinement of the instrumentation is intended by means of electrical noise reduction, signal processing optimization, and transducer development.